made an adjustable piston for the 880

[bhh]

No doubt building a gun from scratch is a huge project.  I'm looking forward to seeing your finished project.

You're right - I need to finish this.

So, what do you think of this idea:

Step one: Drill the valve on each flat like is pictured here. The drill is angled to avoid the intake check valve.:



Step two: Machine a steel bushing with about a .125 wall thickness. (I'll call this my anti-shrapnel bushing.  )  Make the I.D. a slight press fit with the tube O.D. press in place over the valve and secure with green Loctite.  The bushing is represented in this pic with masking tape:



The actual bushing would not extend as far forward as the masking tape. Maybe .50" in front of the valve face.  For that matter, I could make the wall thick enough to hold a pressure gauge and machine a groove for clearance under the barrel and cocking arm.

What do you thinK?

[Rob M]

if everything is locked in place annd u can get good thread engagement on that angle then fine.. It looks challenging from the picture though.. id rather attack the side of the gun for a good drill spot, right through the plastics and into the valve.I LIKE THE LOOKS OF THE BIG HOLE IN THE SIDE.  whatever you are comfortable with. heres the msp p17

[Rob M]

pumping it is plain exausting , pump 1 is easy , pump 2 is the as stock , and pump 3 is incredibly hard. went out back to chrono and the sun was gone too far down.. my caldwell reads anything so i knew it was a lighting issue.. this conversion is not practical.. i was gonna make and sell a few, but it needs an adjustable piston no matter what.. 1/4-20 rod isnt strong enough so i need to change that to 5/16 stainless. Im pretty strong and only 35, I just cant see any old timers enjoying this at all. Not to mention , no matter how u dial the piston , it draws back to far and pops loose when u open the gun fully.. Yes, i can spring load the piston and make it telescopic under load, but again , were back to a lot more work to make it practical.

[bhh]

That's too bad about the P17. I wondered if it would be hard to pump with that short of a lever. I'm a semi-old timer with a bad shoulder so that would be too much for me. I'd still like to see what it's putting out at the chrono, though.

On my valve that big hole is behind the exhaust valve so I can't put an adapter in there.

But, that's not what I was aiming to do. My thought was that the holes would connect the large area between the o-rings with the valve chamber. The sleeve on the outside would increase the thickness of the pump tube so it could contain the pressure safely. So the pump tube between those o-rings become part of the valve. It's like a skeleton valve in a 392.

[Rob M]

On your gun , Im fairly confident the tube wall of the pump would handle whatever the valve will , the reason is adiabatic heating.. while you are pumping , the air in the rear portion of the tube is super hot after a few strokes, so the tube wall pressure is often in excess of what the valve will see ( now you have expansion and cooling post check valve )  So , yes that will work as long as it seals
My p17, its really powerful, i do know that .. chrony results tommorow.. Im hoping it shoots 600fps with 7.9 , i too have a bad shoulder from pumping the dam gun (-'
Lots of people are really interested in this gun being msp , but in order to make it relaible enough for retail , theyd also need to be quite expensive I assume ( money in time , not materials) .. I know stalwart is working on a conversion , but i think he will hesitate to go commerical when he realizes how much strain it puts on the gun chassis

[bhh]

If you're like me, in twenty years you'll be telling the shoulder surgeon you don't know what you did to hurt your shoulder. Lol 

Going commercial: I think it would be tough when you're starting out with a low-cost gun. Most of the cheap stuff is marginally built and a lot of guys don't/can't fit things precisely. By the time I get this NXG shooting hard it will be half worn out.

I see what you mean about the tube already being subject to those pressures. I think I'll give it a shot. At the worst I'll have to build another valve front - or scrap out a $29 rifle.  As I see it, an o-ring seals the two valve halves, so it should seal in this case. They also make some of those super-duper x-rings. Maybe that would work if these don't.

[Rob M]

yep,my shoulders will not last long at this rate.. From what im picturing , all you need to do is drill one small breather hole to take up that vacant volume in the rear portion of the tube.. the stock orings already in place are creating a seal as is.if these orings dont hold, look at the next size up as far as thickness. for example , on the p17 it comes stock with 117 rings., and on the valve wall im using 210 orings ( twice as thick and .015 taller )

[Rob M]

matter of fact i just used this dual oring method like youre planning to seal off the pinning screws on my p17 so the pressured air never sees the screw holes

[bhh]

Good point. I think it will work. I'm using .750 O.D. o-rings instead of the factory metric and it's a tight fit to get them in there.

[Rob M]

sounds good , slight interference is always good , just lube em up real well and make sure the seating location is glass smooth

[Rob M]

im sure u inferred this but there is no need to drill your tube , just the valve for the dual oring seal

[bhh]

Yep! The tube becomes the valve wall.  That's why I was going to put a sleeve on the outside of the tube - it would make the new valve wall stronger.

[Rob M]

ok , just making sure that unless ur installing the gauge , theres no need to drill the tube/ Airguns are hard enough to keep sealed in stock config lol

[bhh]

So, I drilled the valve at two places with a .125" drill. I put the holes on an angle like in my photo.   I ran out of the the oversize 90 Duro SAE o-rings, so I had to use stock metric sizes in 70 Duro.

Results:
No leaks,
No explosion,
10 pumps I'm down about 35fps,
15 pumps I'm about the same,
20 pumps I'm gaining a little ground,
25 pumps and I have a 30 fps gain,
30 pumps and it's at 850fps
from there it slowly improves with a peak of 893 fps at 45 pumps.
I tried for 900fps, but couldn't quite make it - even with 60 pumps. The rifle was getting hot and I was loosing velocity. I lubed the piston o-rings, but it didn't make any difference.

I'm letting it cool now and will try again later. If my arms are up to it. Lol

[Rob M]

hahahahahaaa, thats friggen awesome ! congrats !   so , what id do since u gained sooo much volume , is throw a spacer in there to reduce it back down a bit.. check ur piston orings, make sure all is well lubed.. and try to bore the transfer port another .010.. this way you get to 900 ish in less pumps /./ WAS it loud??

[Rob M]

if you gained ground at 20 pumps , it seems like most of the real compression was done by that point.. i think a few tweaks will have u at 900 inside 30 pumps

[bhh]

hahahahahaaa, thats friggen awesome ! congrats !   so , what id do since u gained sooo much volume , is throw a spacer in there to reduce it back down a bit.. check ur piston orings, make sure all is well lubed.. and try to bore the transfer port another .010.. this way you get to 900 ish in less pumps /./ WAS it loud??

Open transfer port more and play with finding optimal valve volume for 20 and fewer pumps - I was thinking the same thing. 

It was louder, but not loud. I was surprised.

I'm going to have to order more o-rings - I've trashed about 40 of them. Lol

But first I have something else to build:

[Rob M]

sadly , that exact breaking point looks quite familiar.. harbor frieght has entire oring sets in metric and sae for 5.99 , 200 piece. I have several lol.the max, if anything like the 880 should come around 23 pumps, so in reality, if you were getting to 893, you should be able to do it by 25 pumps if all is correct

[Rob M]

since ur gun is rated soo low from the factory, im sure the hammer and trasfer port are slightly restricted not allowing you to dump the full charge quickly..

[bhh]

since ur gun is rated soo low from the factory, im sure the hammer and trasfer port are slightly restricted not allowing you to dump the full charge quickly..

I'm not so sure about that. I think the difference is in the piston. I bought the full-powered version a couple of weeks ago and it had a full length piston. It made about as much power as my first piston did when I added a piece of plastic to take up the headspace. From the factory, mine had about .175" headspace plus a .2x.25 hole in the face of the piston.  I didn't take the rifle apart to check the internals, however, and my son took it with him after a visit, so I can't look at it now.